Speaker
Description
Quantum non-reciprocity, in which quantum-state evolution or photon transport depends on the excitation direction, provides a powerful mechanism for controlling quantum information flow, suppressing back-propagating noise, and preserving coherence in quantum technologies [1]. However, most experimentally demonstrated quantum non-reciprocal effects retain classical analogues [2]. It remained an open question on whether non-reciprocity could be achieved to control the generation of quantum resources themselves, especially at the subwavelength scale for ultimate miniaturization.
Here, we reveal quantum non-reciprocity in photon-pair generation from ultra-thin optical metasurfaces. Photon pairs are key resources for quantum communication, information processing, imaging, and sensing. Nonlinear metasurfaces have recently enabled nanoscale spontaneous parametric down-conversion and tunable entangled photon-pair generation, offering a compact platform for engineering quantum light [3]. Our original theoretical prediction is that the photon-pair rate, quantum state, and entanglement can exhibit non-reciprocity in generation from specially designed metasurfaces.
We perform experimental demonstrations using III–V semiconductor metasurfaces incorporating [110] InGaP nanoresonators. Although the linear transmission remains reciprocal, optical resonances produce direction-dependent internal pump fields, leading to strongly asymmetric spontaneous parametric down-conversion. We show that the difference between the forward- and backward- photon-pair generation rates can be tuned from -15dB to +15dB by varying the metasurface geometry and pump wavelength. Quantum-state tomography further reveals direction-dependent quantum density matrices and entanglement. As a representative example, we observed a fully entangled Bell state for one pump direction, while nearly unentangled for the opposite direction.
We furthermore design metasurfaces incorporating phase-change materials, such as vanadium dioxide, where the generated quantum states and degree of nonreciprocity could be controlled by dynamically tuning the material properties. These results establish a practical route for tailoring quantum non-reciprocity at the nanoscale, establishing new opportunities for compact quantum light sources suited for free-space applications.
$\small{ \mathrm{[1]~S.~Barzanjeh}~\textit{et}~\textit{al},\mathrm{~arXiv~2508.03945~(2025).}}$
$\small{ \mathrm{[2]~ J.-T.~Pan}~\textit{et}~\textit{al},\mathrm{~arXiv~2503.11399~(2025).}}$
$\small{ \mathrm{[3]~ J.~Ma}~\textit{et}~\textit{al},\mathrm{~Advanced~Materials}~\textbf{36},~\mathrm{2313589~(2024).}}$
| I am the presenting author | Yes |
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